Spiral CT

Bushong Chapter 5

Spiral CT

• Continuous source rotation with the patient translation through x-ray beam • Patient couch moves as x-ray tube rotates

Spiral CT

• High voltage supplied by slip rings or on board generator • Slip ring replaces cables • No interscan delay – Interscan delay is a small delay between slices or volumes that is needed during standard axial scanning for the x-ray tube to stop and reverse direction. Interscan delay can also be used to allow extra time during a scan for tube cooling • Volumetric imaging within one breath-hold, at least 25s

Spiral CT

• Hyperventilation may help patients extend breath-hold – Tell the patient to breath in and out several times before final breath hold • Contrast enhanced examination requires less contrast media – Contrast amounts between 100-150ml are still common – Some multislice/multidetector scanners are so fast that technologists need to be careful about not scanning faster than the contrast bolus moves through the vascular system • Detector data transferred to computer by slip rings

Spiral CT

• Examination time is greatly reduced • Patient comfort is much improved • Because the patient is moved through the gantry while the x-ray tube rotates, a spiral pattern results

Spiral CT

• Z-axis resolution is slightly reduced with spiral CT • Effective slice thickness increases with pitch

Pitch

• Pitch is the patient couch movement per rotation divided by slice thickness • Contiguous spiral – Pitch = 1, that is 10mm/10mm • Extended spiral – Pitch = 2, that is 20mm/10mm • Overlapping spiral – ½, that is 5mm/10mm • Page 78 Bushong shows diagram

Pitch

• Low pitch results in better z-axis resolution • Narrow collimation/high pitch results in better z-axis resolution than wide collimation/low pitch • Narrow collimation/high pitch is recommended for high contrast, thin slice examination, for example, lung nodules

Pitch

• Data is collected continuously but not from a transverse plane – Instead a helical volume is created • As spiral CT pitch increases, patient dose is reduced • Patient dose is approximately proportional to 1/pitch • Patient dose is proportional to slice thickness divided by couch movement

Pitch

• Pitch in excess of 2 is not recommended for any clinical examination – Too much anatomy would be skipped • Couch incrementation is usually set to equal collimation, pitch = 1 • Couch speed (mm/s) should not exceed slice thickness (mm) in order to obtain best compromise between image quality and image volume

Pitch

• Couch speed will not normally exceed 10mm/s • When pitch exceeds 1, 180 interpolation must be used to limit loss of z-axis resolution • Longitudinal (z-axis) image coverage is the product of couch velocity (mm/s) and image time (s) – If couch moves 10mm/s and the imaging time is 5s then 50mm (5cm) of axial coverage will result • The larger the pitch, the more anatomy is covered per examination

Interpolation

• Reconstruction of spiral CT images is the same as that for conventional CT except for interpolation • A transverse planar image can be reconstructed at any position along the axis of the patient (z-axis) • The transverse image is reconstructed from spiral data first by interpolation, then by filtered back projection

Interpolation

• Either 360 degrees or 180 degrees interpolation may be employed • Usually 180 degree interpolation is preferred • Contiguous reconstruction can result in partial volume effect when object is contained in adjacent slices

Interpolation

• Overlapping reconstruction may be necessary to ensure that object is fully contained within a slice • Data acquisition is continuous along the z-axis; therefore by interpolation, image reconstruction is at any z-axis position • Regardless of z-axis position, slice thickness is determined by collimation

Interpolation

• Volume averaging increases with increasing pitch • By increasing the pitch the helix becomes more spread out and small structures can be missed • This is similar to stretching a slinky toy

Interpolation

• Image noise is higher with spiral CT versus conventional CT regardless of pitch • Interpolation is the computation of an unknown value using known values on either side • Z-axis resolution is improved with 180 degree interpolation compared to 360 degree interpolation

Interpolation

• Extrapolation is the computation of an unknown value using known values on one side • 180 degree interpolation results in a thinner slice than 360 degree interpolation • 180 degree interpolation results in a noisier image than 360 degree interpolation

Interpolation

• 180 degree interpolation results in approximately 20% higher noise than conventional CT • 360 degree interpolation results in approximately 20% less noise than conventional CT • 180 degree interpolation results in better z-axis resolution on reformatted longitudinal images than 360 interpolation

Interpolation

• 180 degree interpolation allows scanning at a higher pitch than 360 degree • 360 interpolation broadens sensitivity profile more than 180 degree interpolation • In general, image noise is less for 360 degree interpolation, spiral CT than for conventional CT • In general, image noise is much higher for 180 degree interpolation, spiral CT than for conventional CT

Interpolation

• Whether 180 degree or 360 degree interpolation, there are linear and higher order reconstruction algorithms • Two characteristic spiral CT artifacts have been identified as breakup and stair step • Both the breakup artifact and the stair step artifact occur as a consequence of reformatting interpolation transverse images to the longitudinal plane – coronal or sagittal

Sensitivity Profile

• Generally when covering a given length of anatomy, thinner collimation and higher pitch are preferred because the result is better spatial resolution • Pitch greater than 2:1 is not clinically useful because of a broadened sensitivity profile and reduced z-axis resolution • Generally, higher pitch results in thinner slice thickness and less partial volume artifact

Sensitivity Profile

• During spiral CT with pitch >1, the sensitivity profile (z-axis resolution) is wider than that of conventional CT • Spiral CT sensitivity is described by the full width at tenth maximum (FWTM) rather than the conventional full width at half maximum (FWHM) • The higher the pitch, the wider will be the sensitivity profile

Design Features – Slip Ring Technology

• Slip ring technology made spiral CT possible • Normal spiral CT gantry rotation is 1 revolution per second • Although .5s revolution is possible, the engineering required by the stress of centrifugal force is formidable

Slip Ring Technology

• There may be multiple slip rings, both high voltage and low voltage • The design of spiral CT imagers is based on both third and fourth generation with no clear advantage to either • The slip ring contacts or brushes wear and are designed to be replaced during preventive maintenance

X-Ray Tube

• Spiral CT requires less than 1s 360 degree rotation time and at least 5 MHU x-ray tubes • For very long scan times, mA must be reduced so that x-ray tube loading will not be exceeded • Regardless of heat capacity (MHU) and anode cooling (kHU/min), spiral CT is usually limited by the heat capacity of the focal track • High anode heat capacity (6-8 MHU) and rapid cooling (1 MHU/min) are required

X-Ray Tube

• In spite of the high heat load, tube life is comparable to conventional CT at about 50,000 exposures

Technique Selection Reconstruction

• Index is the interval at which images are reconstructed • Index is reconstruction distance divided by collimation • An index of less than one indicates image overlap • An index of greater than one indicates a gap

Reconstruction

• An index of less than one should be employed to visualize suspected lung nodules • Spiral CT significantly improves coronal and sagittal slice reconstruction • High quality two-dimensional and three dimensional image reformation are made from overlapping transverse images

Reconstruction

• Spiral CT operation requires the following unique technique selections – Scan time – Beam collimation – Couch feed velocity – Z-axis spacing for image reconstruction • Spiral images cannot be reconstructed as rapidly as they are acquired. Hence computer memory must be excessive

Reconstruction

• Scan time cannot exceed the patient’s breath-hold capacity, usually about 25s • Collimation and couch velocity can be selected as pitch

Z-Axis Resolution

• Z-axis resolution is compromised in spiral CT but not significantly • The ability to reconstruct images at any z-axis location improves small lesion detection by reducing partial volume effects

Advantages and Limitations

• Image noise is usually less with spiral CT • More data is acquired in spiral CT; therefore, image reconstruction takes a little longer • Spiral CT replaces single scan techniques with volume acquisition techniques • Spiral CT misses no anatomy in the scanned volume

Advantages and Limitations

• Spiral CT images can be reconstructed at any z-axis position • Multiple overlapping transverse images are possible in a single breath-hold with no additional patient dose • Overall scan time is less with spiral CT resulting in improved patient throughput • Spiral CT takes a bit longer for image processing because of the required interpolation before planar image reconstruction

Advantages

• Faster image acquisition • Contrast can be followed quicker • Reduced patient dose at pitch > 1 • Physiologic imaging • Improved 3d imaging • Angiographic imaging • Fewer partial volume artifacts • Freeze breathing • Fewer motion artifacts • No misregistration • Increased throughput • Improved patient comfort • Unlimited z-axis resolution • Real time CT biopsy